Fix is_list_cyclic

[gmstanle]

This PR fixes the testing of is_list_cyclic so that it tests for correctly identifying the start of the cycle. This can be verified with the following solution code (SPOILER alert for those who have not solved it):

This solution will succeed on the old code but fail on the PR

def has_cycle(head: ListNode) -> Optional[ListNode]:
    """Return None if there is no cycle, or the node at the start
    of the cycle if there is a cycle."""
    
    # If list has only 1 element, no cycle.
    if not head:
        return None
    if not head.next:
        return None
    
    # Slow and fast node.
    slow, fast = head, head
    i = 0
    while fast.next and fast.next.next:
        # Move fast up every round.
        fast = fast.next
        # Move slow up every other round.
        if i % 2:
            slow = slow.next
        
        # Detect cycle and return slow - this is incorrect but passes current tests
        if fast is slow or fast.next is slow:
            return slow
        
        i += 1
    
    # If we reach the end (fast.next is None).
    return None
    ```
    
    This code was NOT LLM-generated.
    

[gmstanle]

This correct code will pass the updated tests:

```
def has_cycle(head: ListNode) -> Optional[ListNode]:
"""Return None if there is no cycle, or the node at the start
of the cycle if there is a cycle."""

# If list has only 1 element, no cycle.
if not head:
    return None
if not head.next:
    return None

# Slow and fast node.
slow, fast = head, head
i = 0
while fast.next and fast.next.next:
    # Move fast up every round.
    fast = fast.next
    # Move slow up every other round.
    if i % 2:
        slow = slow.next
    
    # Detect cycle
    if fast is slow or fast.next is slow:
        # We have a cycle!
        # We can count the length of the cycle in O(n) time by
        # freezing slow and moving fast until it hits slow again.
        fast = slow.next
        cycle_length = 1
        while fast is not slow:
            fast = fast.next
            cycle_length += 1
        print(cycle_length)
        
        # Now that we have cycle length, we just start fast and slow at head.
        # increment slow after incrementing fast cycle_length times. once 
        # fast is slow, slow is the start of the cycle.
        fast, slow = head, head
        for _ in range(cycle_length):
            fast = fast.next
            
        while fast is not slow:
            fast = fast.next
            slow = slow.next
        
        return slow
    
    i += 1

# If we reach the end (fast.next is None).
return None
```